Hot die printing is a technique for embossing a workpiece of metal or plastic in a stamping press with a print medium such as leaf foil or carbon black. The workpiece may be either a discrete element such as a plastic identification tag, or a section of a continuous element, such as a length of metal tubing. Usually, a line of type is held by an electrically heated type holder attached to the ram or head of the stamping press. The workpiece, e.g., a laminated plastic identification card presented to the stamping press operator for validation, is positioned on an anvil under the line of type while the print medium is arranged between the line of type and the workpiece. The ram is then lowered, causing the hot line of type to press the print medium against the workpiece. After a short interval, the ram is raised, the embossed workpiece is removed, the print medium advanced, and the technique repeated with another
Precision and operator safety are two competing requirements in hot die printing. Precision is required because the location to be embossed on the workpiece is often only a few square millimeters in area; if the workpiece is improperly positioned under the ram the line of type may extend over the edge of the workpiece and make an incomplete impression or overlap into an adjacent location and obliterate a previous impression. Either result is unacceptable, particularly where the attempted impression is sought for identifying the workpiece or the bearer of the workpiece. Efforts for consistent precision in hot die printing usually mean that the stamping press operator manipulates the workpiece while the location to be embossed is approximately aligned in the confined space between the anvil and the ram. For small workpieces, this necessarily exposes the operator to the hazard of touching the hot (200.degree. to 600.degree. F.) print holder, a manifestly undesirable risk to personnel safety.
One exemplar of the type of stamping press used in the prior art is disclosed in Kingsley, U.S. Pat. No. 2,725,817. A fixture attached to a base plate holds a continuous length of workpiece material such as wire rod or tubing in position under the ram of the press. A guide aligns a segment from a roll of print medium between the workpiece and the ram. The fixture has a central channel containing an anvil tailored to the cross-sectional dimension of the workpiece. A central rib runs the length of the anvil. Roller bars disposed above cross channels at either end of the central restrain movement of the material.
Another exemplar is disclosed in the Kingsley U.S. Pat. No. 3,135,198. The stamping press described uses a fixture for holding a continuous length of plastic tape under a heated ram. The guide has a central channel holding a fixture which also contains a central channel. The central channel of the fixture has a width equal to that of the soft material and is partially filled with a cushion.
The chief disadvantage of the workpiece guides and fixtures used in the prior art stamping presses is their unsuitability for accurately positioning discrete workpiece items under the ram. Consequently, hot die stamping press operators are forced to use small tweezers or pliers to align the workpiece, a step that is both time consuming and hazardous. Efforts to use a prior art workpiece guide such as that disclosed in the Crafts, et al. U.S. Pat. No. 131,152, for hot die printing would be fruitless because that style of guide relies upon manual placement of a tool to position the workpiece.